This invention relates to tandem parking brake calipers for disc brakes. Moreover, this invention relates to suck a parking brake caliper which is added to, but which functions separately and independently of, the existing service brake of the vehicle wheel.
Parking brakes for automobiles are well known and have been widely used in the art. The continuous development of the automotive industry has made it essential that the development of hydraulic brakes embark upon new courses. Accordingly, modern vehicles now incorporate disc brakes on at least one pair of the vehicle wheels. These disc brake systems incorporate a generally circular disc-like member called a rotor which presents parallel outer surfaces. The parallel surfaces engage friction linings of the service brake caliper which, in turn, surrounds the circumferentially outer peripheral portion of the rotor. The service brake provides dynamic stopping of the vehicle during normal vehicle operation. The service brake is applied by a brake pedal or a pedal-actuated booster piston. In both cases, the vehicle operator initiates the action to force the friction linings within the caliper against the disc rotor. When the operator is out of the vehicle and the vehicle is parked, such as, for example, on a hill, means must be applied to hold the vehicle stationary without the continued assistance of an operator. Thus, in the prior art, parking brakes have been provided. In more recent systems, these parking brakes have taken one of two forms. In the first, a so-called drum-in-hat brake is provided in which the service disc brake incorporates a small drum brake assembly inside of the cylindrical portion of the existing rotor. This small drum brake is used for parking purposes only and is actuated by a cable. The second prior art parking brake device for modern disc brake systems is referred to as an integral adjusting caliper which incorporates a mechanical actuator and an adjuster inside of the service caliper bore. Both of these known systems exhibit significant problems as will be appreciated by those skilled in the art.
For example, the drum-in-hat parking brakes have the disadvantage that a complete second drum brake assembly must be provided in addition to the service caliper brake. Further, these parking brakes are susceptible to corrosion problems. Still further, to service the drum-in-hat parking brake, it is ordinarily necessary to remove the rotor and the caliper from the wheel as well as the various drum brake components. It also has been found that it is difficult to mount these parking brakes in the wheel-end environments of many modern-day vehicles.
The integral adjusting power brakes have disadvantages in that they require costly gears and bearings and in that their overall performance is limited because the brake holding function relies upon the frictional values of the linings of the service brake. That is, parking brakes typically are not power-assisted by means of a booster and rely, instead, upon a hand-operated parking brake cable and lever. Thus, the relatively low friction values of the service brake linings, while suitable for dynamic, power-assisted braking, has not been sufficient to provide the same brake-holding power in parking brake applications. Further, a failure of the integral adjusting devices may also disadvantageously result in a failure of the entire service brake as well. This is a problem. Still further, the integral adjusting parking brake has been found to be too complicated to service in the field.
These and other disadvantages are overcome by the present invention wherein a tandem parking brake caliper is provided wherein no "automatic adjustment" is needed and only a minimum number of low-cost additional components are required. Further, the caliper utilizes unique maximum friction linings which are provided at a larger effective radius near the outside diameter of the rotor. The device is easy to service as a minimum number of parts are utilized and the overall packaging can be provided at any one of a number of convenient locations about the wheel. Further, the linings of the tandem parking caliper, in accordance with the teachings of the present invention, should never wear out unless, of course, the parking caliper is repeatedly utilized as an emergency brake in the event of failure of the service brake. That is, the unique high friction linings normally engage the rotor in a fixed static manner and therefore are not subjected to wear. Further, the tandem parking caliper in accordance with the present invention advantageously remains fully functional in the event of failure of the existing service caliper; and thus truly provides an emergency brake.